Thin film photovoltaic solar panels are generally used to harvest solar radiation to produce electricity and typically comprise a plurality of thin films of material deposited on a sheet of glass. The panels may comprise an effective panel area of several square meters, in which case, a panel with a 15% efficiency which receives 1000 W/m2 will generate several hundred Watts of electrical power. In situations where the operating voltage is 0.6V, this would relate to an operating current of several hundred amperes, and thus generate unacceptable resistive losses in the relatively thin conductive films. As a result, it is common to divide or scribe the panel into individual devices having a width of only a few hundred microns, to reduce the effective area and thus reduce the current flowing in each device. The individual devices can then be electrically connected in series, and electrical power extracted at higher voltage and reduced current, thereby significantly reducing resistive losses.
Referring to FIG. 1 of the drawings, thin film photovoltaic devices are commonly formed by depositing a layer of transparent conductive oxide (TCO) 11 such as tin oxide or tin doped indium oxide, upon a glass sheet (FIG. 1a) 10. The TCO layer 11 is subsequently scribed to create strips 11a of TCO which extend across the glass sheet (FIG. 1b). This scribe is commonly referred to as a P1 scribe. Once the P1 scribe has been performed a radiation absorbing layer 12, such as amorphous silicon (a-Si) is deposited upon the TCO strips 11a and within the grooves 11b between the TCO strips 11a formed by the scribe (FIG. 1c). The a-Si layer 12 is subsequently scribed to form strips 12a which similarly extend across the glass sheet 10, and which are separated by grooves 12b which terminate on the TCO strips. This is commonly referred to as a P2 scribe (FIG. 1d). The P2 scribes formed in the a-Si layer are arranged parallel to the P1 scribes, but displaced laterally across the sheet, from them.
Following the P2 scribe through the a-Si layer, a metal layer 13 is deposited upon the a-Si layer and serves as a device back contact (FIG. 1e). The back contact 13 may be formed using silver, gold, copper, titanium, aluminium, indium, platinum or similar metals and/or their alloys. The back contact layer 13 is subsequently scribed to form strips 13a which are separated by grooves 13b, which extend through the a-Si layer to the TCO layer (FIG. 10. This is commonly referred to as a P3 scribe. The P3 scribes are formed substantially parallel to the P1 and P2 scribes but laterally displaced across the sheet, from them. P3 scribes typically involve removal of both radiation absorbing material as well as back contact material. A final scribe process, namely the so-called isolation scribe, may then be employed to electrically isolate the separate devices on the panel.
The P1 scribing process is typically performed using laser pulses from Q-switched mode-locked diode pumped solid state lasers, operating at 1064 nm with pulsewidths in the range of 10 ns to 200 ns and pulse repetition frequencies in the range of 50-150 kHz. Unfortunately, as those skilled in the art will recognize, nanosecond laser scribing of TCO materials employed in a-Si photovoltaic devices typically suffers from substantial heating and related micro-cracking which can compromise device performance, leading to premature device failure and consequently a higher unit price per device.
Mode-locked, diode pumped solid state lasers are well-known in the art for producing relatively high output powers with pulse widths less that 15 ps. The reduced pulse duration is found to reduce the heating of the TCO; however, such lasers typically produce a very narrow pulsewidth range due to the limitations of the mode-locking methods employed to generate the picosecond seed pulses produced by the oscillators in these devices. Furthermore, as is well known to those skilled in the art, substantially varying the pulse repetition frequency typically produces large variations in the key operating characteristics of the laser, such as beam propagation values and pulse stability, both of which can be very important in laser scribing, especially in a production setting.
Picosecond mode-locked diode pumped solid state lasers typically contain many free-space coupled optics, which are difficult to maintain in proper alignment. Furthermore, such laser systems are known to be relatively expensive in comparison to nanosecond diode pumped solid state lasers and their cost remains a barrier to wide industrial application.
Numerous workers have described the prior art employment of fixed pulsewidth nanosecond pulses employing Gaussian optics for laser processing of specimen films employed in a-Si photovoltaic devices. U.S. Pat. No. 4,292,092 describes employment of pulsed output from a Q-switched Nd:YAG laser emitting at 1064 nm for the formation of P1, P2, and P3 scribes. More recently, in “Recent Advances in Laser Scribing Process Technologies for Thin Film Solar Cell Manufacturing,” Bonse et al describe employment of a 1064 nm Spectra-Physics HIPPO Model H10-106Q diode-pumped Q-switched Nd:YVO4 laser to produce a 40 μm wide scribe in an indium tin oxide P1 layer by employing an 18 ns pulse, 20 W average laser power output at a pulse repetition frequency of 80 KHz (250 μJ/pulse) at a scribing speed of about 1.25 m/s. Bonse also describes producing 38 μm P2 scribe widths in 220 nm thick a-Si layer, by employing 532 nm, 35 ns pulsed output at pulse repetition frequency of 60 KHz from a diode-pumped Q-switched frequency doubled Nd:YVO4 Spectra-Physics Explorer laser, at scribing speeds ranging from about 0.9 m/s to about 1.9 m/s.